1. Field of the Invention
The present invention relates to a zoom lens having an image stabilizing function for use in an image pickup apparatus, such as a single-lens reflex camera, a digital camera, a video camcorder, or a film camera.
2. Description of the Related Art
As a photographing optical system included in an image pickup apparatus (e.g., digital camera or video camcorder), a zoom lens is desired to be compact, have a high zoom ratio, and provide high imaging performance throughout the entire zoom range.
As a zoom lens with a wide zoom range and a high zoom ratio, there is known a telephoto zoom lens having a long focal length at the telephoto end. The telephoto zoom lens tends to be large in overall size and weight.
When a zoom lens is inclined due to vibration, the imaging position is displaced (or the photographed image is blurred) by an amount according to the angle of the inclination and a focal length corresponding to the zooming position at the time when the zoom lens was inclined.
Therefore, in the telephoto zoom lens described above, hand-induced shake (or vibration in the zoom lens) during photographing can cause significant blur in the photographed image.
Telephoto zoom lens having an image stabilizing function is known. In such a telephoto zoom lens, at least one of the lens units is moved in a direction substantially perpendicular to the optical axis so as to compensate (or correct) for image blur caused by vibration.
Among zoom lenses of this type is a four-unit zoom lens which includes four lens units having positive, negative, positive, and positive refractive power in order from the object side to the image side, and in which a lens subunit constituting part of the third lens unit is moved in a direction perpendicular to the optical axis to compensate for image blur (see, for example, U.S. Pat. No. 7,253,965 and U.S. Pat. No. 6,046,852).
Also, there is known a five-unit zoom lens which includes five lens units having positive, negative, positive, negative, and positive refractive power in order from the object side to the image side, and in which the fourth lens unit is moved in a direction perpendicular to the optical axis to compensate for image blur (see, for example, U.S. Pat. No. 6,025,962).
There is also known a six-unit zoom lens which includes six lens units having positive, negative, positive, positive, negative, and positive refractive power in order from the object side to the image side, and in which the fifth lens unit is moved in a direction perpendicular to the optical axis to compensate for image blur (see, for example, U.S. Pat. No. 7,068,428 and U.S. Pat. No. 6,266,189).
In such a zoom lens with an image stabilizing function, it is generally desired that the achieved amount of image blur correction be large, and that the required amount of movement of an image stabilizing lens unit for correcting (or compensating) for image blur be small. If the lens arrangement in the image stabilizing lens unit, which is moved for image stabilization, is not appropriate, the amount of eccentric aberration (i.e., aberration caused by eccentricity) that occurs during image stabilization increases. This causes degradation in optical performance during image stabilization. Therefore, for a zoom lens with an image stabilizing function, it is desired that the amount of eccentric aberration that occurs during image stabilization be small. At the same time, it is desired that a significant amount of image blur correction be achieved with a small amount of movement of the image stabilizing lens unit. In other words, it is desired to achieve a high level of vibration control sensitivity, which is the ratio of the amount of image blur correction (ΔX) to the unit amount of movement of the image stabilizing lens unit (ΔH), ΔX/ΔH.
In a telephoto zoom lens with a high zoom ratio, a larger amount of eccentric aberration tends to occur on the telephoto side during image stabilization, and it is difficult to correct such aberration.
Therefore, for a telephoto zoom lens having an image stabilizing function, it is important to reduce the amount of eccentric aberration during image stabilization by choosing an appropriate overall lens arrangement and an appropriate lens arrangement in the image stabilizing lens unit, which is moved for image stabilization.
In particular, eccentric aberration that occurs in the image stabilizing lens unit is formed on the image plane at the imaging magnification of a lens unit arranged on the image side of the image stabilizing lens unit.
Therefore, for a telephoto zoom lens having an image stabilizing function, it is important to appropriately configure the image stabilizing lens unit and the lens unit arranged on the image side of the image stabilizing lens unit.